Conventionally, grain-oriented silicon steel is manufactured by the following process, wherein:
Steel is secondarily refined and alloyed in a converter (or an electric furnace), and then continuously cast into slab, the basic chemical composition of which includes Si (2.5-4.5%), C (0.01-0.10%), Mn (0.03-0.1%), S (0.012-0.050%), Als (0.01-0.05%) and N (0.003-0.012%), in some instances further comprising one or more elements of Cu, Mo, Sb, Cr, B, Bi and the like, balanced by iron and some unavailable inclusions;
The slab is heated to about 1400° C. in a special-purpose high-temperature heater and kept at this temperature for more than 30 minutes to sufficiently solid dissolve favorable inclusions, so that dispersed fine particles of secondary phase, namely inhibitor, precipitate in the silicon steel matrix during subsequent hot rolling; after or without normalization, the hot rolled sheet is scrubbed with acid to remove iron scale from its surface; the sheet is rolled to the thickness of the final product with single cold rolling or more than two cold rollings with annealing therebetween, coated with an annealing separator comprising MgO as the main component, and then decarburizing annealed to lower [C] in the steel sheet to a level not influencing the magnetism of the final product (typically lower than 30 ppm); physical and chemical changes such as secondary recrystallization, formation of Mg2SiO4 underlying layer, purification (for removing elements harmful to magnetism, such as S, N, etc. in steel) and the like occur in the steel sheet during the high-temperature annealing process, giving grain-oriented silicon steel with high orientation and low iron loss; finally, after coated with insulating coating, stretched and annealed, grain-oriented silicon steel product ready for commercial use is obtained.
Conventional grain-oriented silicon steel exhibits the following notable characteristics:
(1) Since inhibitor is formed at the very beginning of the refining of steel and functions in subsequent procedures, it has to be controlled and regulated;
(2) The temperature up to 1400° C., at which the slab is heated, reaches the limit of a conventional heating furnace, and the control capability on the temperature drop of a rolling line also arrives at the limit of existing hot rolling technologies;
(3) The key of the production process is the control of the microstructure and texture of the steel sheet in each stage, and the behavior of the inhibitor;
(4) Heating at high temperature results in low utility of the heating furnace which needs frequent repair, high burning loss, large energy consumption, and severe edge cracking of the hot rolled coil, leading to difficulty in cold rolling procedure, low yield and high cost.
After half a century's development, the production technology of high-temperature grain-oriented silicon steel is well established and produces top-grade grain-oriented silicon steel products, contributing a lot to the development of electric and electronic industry. However, due to complicated production process, high technicality, serious inter-enterprise technical blockade, as well as special, narrow use of the technology and thus low total demand of the products, this technology is mastered by only a few steel manufacturers. On the other hand, heating at high temperature brings about a series of problems, for example, the need of special-purpose high-temperature heating furnace, poor practicality in production, high cost and the like.
In an attempt to solve these problems, some methods have been tried and developed successfully in long-time practice of production and research, which are described as follows.
(1) Method Using Electromagnetic Induction Heating
The method using electromagnetic induction heating, practiced by Nippon Steel Corp. and Kawasaki Steel Corp., is essentially one that heats slab at high temperature, except that, at the stage of heating slab at high temperature, N2 and H2 are introduced into the electromagnetic induction heating furnace as protective gases to control the atmosphere precisely, so that high-temperature oxidation of the slab is inhibited. Meanwhile, the fast heating rate in this method shortens the time for maintaining the furnace at high temperature. This method has solved the problem of edge cracking to a great extent. Specifically, an edge crack may be reduced to less than 15 mm, improving the producibility of grain-oriented silicon steel. Unfortunately, edge cracking can't be eliminated completely.
(2) Method for Producing Grain-Oriented Silicon Steel at Medium Temperature
A technology for producing grain-oriented silicon steel at medium temperature is adopted by VIZ, Russia, etc., wherein slab is heated at 1250-1300° C., the content of Cu in the chemical composition is relatively high, and AlN and Cu act as inhibitors. Similar to the case in the high-temperature method, the inhibitors herein are inherent too. The problem of edge cracking incurred by heating at high temperature may be avoided entirely in this method. However, as a drawback, this method can only be used to produce common grain-oriented silicon steel, rather than high magnetic induction grain-oriented silicon steel.
(3) Method for Heating Slab at Low Temperature in Japan
According to this method, slab is heated at a temperature lower than 1250° C., leading to no edge cracking and good producibility of hot rolled sheet. The inhibitors herein are acquired inhibitors, obtained by nitridation after decarburizing annealing. Thus, this method may be used to produce both common grain-oriented silicon steel and high magnetic induction grain-oriented silicon steel.
(4) CSP Method for Producing Oriented Silicon Steel
This method has also tackled the problem of edge cracking during hot rolling oriented silicon steel, improving producibility while lowering production cost. The inhibitors herein are acquired ones too, obtained by nitridation.
It is obvious that heating slab at low temperature stands for the developmental trend of the technology for producing grain-oriented silicon steel, for it overcomes the innate drawback suffered by heating slab at high temperature, improves producibility and lowers cost.
For example, a method for producing grain-oriented silicon steel at low temperature in Japan is described in Japanese Patent Publication Heisei 3-211232. In this patent, chemical composition 1 comprises [C] 0.025-0.075%, Si 2.5-4.5%, S≦0.015%, Als 0.010-0.050%, N≦0.0010-0.0120%, Mn 0.05-0.45%, Sn 0.01-0.10%, balanced by Fe and unavailable inclusions. After heated at a temperature lower than 1200° C., the slab is hot rolled, and then rolled to the thickness of the final product with single cold rolling or more than two cold rollings with annealing therebetween at a cold rolling reduction rate of over 80%. Subsequently, the resultant sheet is decarburizing annealed and high-temperature annealed, during which nitridation is carried out once secondary recrystallization begins.
Chemical composition 2 comprises [C] 0.025-0.075%, Si 2.5-4.5%, S≦0.015%, Als 0.010-0.050%, N≦0.0010-0.0120%, B 0.0005-0.0080%, Mn 0.05-0.45%, Sn 0.01-0.10%, balanced by Fe and unavailable inclusions. After heated at a temperature lower than 1200° C., the slab is hot rolled, and then rolled to the thickness of the final product with single cold rolling or more than two cold rollings with annealing therebetween at a cold rolling reduction rate of over 80%. Subsequently, the resultant sheet is decarburizing annealed and high-temperature annealed, during which nitridation is carried out once secondary recrystallization began.
After decarburizing annealing, oxygen content of the steel sheet may be converted to that of a 12 mil sheet: [O]ppm=55t±50 (t: sheet thickness in mil). This method may be used to produce high electromagnetic induction grain-oriented silicon steel.
In a method described in Japanese Patent Publication Heisei 5-112827, the chemical composition comprises [C] 0.025-0.075%, Si 2.9-4.5%, S≦0.012%, Als 0.010-0.060%, N≦0.010%, Mn 0.08-0.45%, P 0.015-0.045%, balanced by Fe and unavailable inclusions. After heated at a temperature lower than 1200° C., the slab is hot rolled, and then rolled to the thickness of the final product with single cold rolling or more than two cold rollings with annealing therebetween. After decarburizing annealing, the resultant sheet is continuously nitrided while it advances. After coated with a separator, it is annealed at high temperature, producing grain-oriented silicon steel having good magnetism and underlying layer quality. In the nitriding process, the protective atmosphere is a gas mixture of H2 and N2, the content of NH3 is over 1000 ppm, the oxygen potential is pH2O/pH2≦0.04, and the nitriding temperature is 500-900° C.
During high-temperature annealing, the atmosphere is kept weakly oxidative at 600-850° C.
In a method of Acciai Speciali Terni Spa for producing grain-oriented silicon steel at low temperature as described in Chinese Patent CN1228817A, the chemical composition comprises Si 2.5-5%, C 0.002-0.075%, Mn 0.05-0.4%, S (or S+0.503Se)<0.015%, acid soluble Al 0.010-0.045%, N 0.003-0.013%, Sn≦0.2%, balanced by Fe and unavailable inclusions. The steel of the above composition is cast into thin slab, which is then heated at 1150-1300° C. After hot rolling, the slab is normalizing annealed and subjected to final cold rolling at a reduction rate of 80%. When final high-temperature annealing is carried out, the annealing atmosphere is controlled to keep the content of absorbed nitrogen by the steel lower than 50 ppm. This method doesn't use nitriding process, mainly suitable for producing grain-oriented silicon steel by continuously casting thin slab.
In a method disclosed in Chinese Patent CN1231703A, the chemical composition is a low carbon system containing copper. The production process is substantially consistent with the forgoing patent except that the steel sheet is nitrided at 900-1050° C. at a nitriding amount of less than 50 ppm after decarburizing annealing. This method is suitable for the production of grain-oriented silicon steel from thin slab.
In another method disclosed in Chinese Patent CN1242057A, the chemical composition comprises Si 2.5-4.5%; C 150-750 ppm, most preferably 250-500 ppm; Mn 300-4000 ppm, most preferably 500-2000 ppm; S<120 ppm, most preferably 50-70 ppm; acid soluble Al 100-400 ppm, most preferably 200-350 ppm; N 30-130 ppm, most preferably 60-100 ppm; Ti<50 ppm, most preferably less than 30 ppm, balanced by Fe and unavailable inclusions. Slab is heated at 1200-1320° C. and nitrided at 850-1050° C. The other procedures are substantially the same as the above two patents.
Still another method disclosed in Chinese Patent CN1244220A features simultaneous nitridation and decarburization.
The key point of other patents is the existence of precipitated dispersed phase in hot rolled sheet, facilitating high-temperature nitridation at 900-1000° C. It may be summarized that the low-temperature technology of Acciai Speciali Terni Spa is limited to high-temperature nitridation and/or production of grain-oriented silicon steel by continuously casting thin slab. The main point lies in the existence of precipitated dispersed phase in hot rolled sheet, which is favorable for high-temperature nitridation that is carried out concurrently with or after decarburization.
The chemical composition of the low-temperature grain-oriented silicon steel developed by POSCO, South Korea, comprises C 0.02-0.045%, Si 2.9-3.30%, Mn 0.05-0.3%, acid soluble Al 0.005-0.019%, N 0.003-0.008%, S<0.006%, Cu 0.30-0.70%, Ni 0.30-0.70%, Cr 0.30-0.70%, balanced by Fe and unavailable inclusions. In addition, the steel comprises 0.001-0.012% B. Decarburization is carried out at the same time with nitridation which occurs in moisture atmosphere. The basis of this method is the use of BN as the main inhibitor.
The methods described in Chinese patents such as Nos. 85100664 and 88101506.7 are all based on the conventional process wherein inhibitors are solid dissolved during heating and precipitation is controlled during rolling. The heating temperature actually approximates 1300° C., essentially different from the method of the present invention. The method described in Chinese Patent ZL200410099080.7 to Baosteel features nitridation before decarburization.
After consulting and analyzing relevant patents, references and the like on the technologies for producing grain-oriented silicon steel by heating slab at low temperature according to a nitriding process, it may be found that Japanese technologies focus on nitridation of steel sheet during the period from the end of decarburizing annealing to secondary recrystallization, and on the formation of inhibitors at the early stage of high-temperature annealing; European technologies are characterized by nitridation after or at the same time with decarburizing annealing, and by high nitriding temperature; POSCO technology is suitable for a composition system containing low carbon and low Al, wherein nitridation and decarburization are carried out concurrently.
When Japanese nitriding processes are used to produce grain-oriented silicon steel, growth of crystal grains formed during primary recrystallization can't be prevented due to the absence of inhibitors in steel sheet. The size of the crystal grains formed during primary recrystallization is controlled mainly by temperature and time. Thus, there is a high demand on the control of decarburizing annealing and nitriding process, and the process window is narrow. On the other hand, an oxide layer with SiO2 as the main component has already formed on the steel sheet surface before nitridation is carried out after decarburizing annealing, so that the consistency and behavior of nitridation are liable to the interference of the oxide layer on the surface. The Acciai Speciali Terni Spa technology features high-temperature nitridation. To effect this process, slab has to be heated at a relatively high temperature, for example, about 1250° C., so that dispersed particles of second phase precipitate in hot rolled sheet as desired. Thus, favorable inclusions in the hot rolled sheet have to be controlled. In addition, nitridation is carried out after or at the same time with decarburizing annealing. POSCO also adopts the process wherein decarburization and annealing are carried out concurrently. As a result, the oxide layer on the steel sheet surface has an unavailable impact on nitridation. Furthermore, the steel has a low content of Al, and BN is the main inhibitor. The instability of B will render the inhibiting capability of the inhibitor unstable, and the stability of magnetism will be affected to a great extent.
Table 1 compares the chemical composition systems of grain-oriented silicon steel produced by several technologies for heating slab at low temperature.
TABLE 1Comparison among chemical composition systems unit: wt. %CSi MnPSNAlsCuSnBNiCrJapan 0.025-2.5- 0.05-0.015-≦0.0150.0010-0.010-/0.01-0.0005-//0.0754.50.45 0.0450.01200.0500.100.0080AST0.002-2.5-0.05-/≦0.0150.003-0.010-/≦0.2///0.07550.40.0130.045POSCO0.02-2.9-0.05-/<0.0060.003-0.005-0.30-/0.001-0.30- 0.30-0.0453.30 0.30.008 0.0190.700.0120.70 0.70The0.035-2.9- 0.08- 0.010-0.005-0.005- 0.015-0.05-0.001-//≦0.2invention0.0654.0 0.18 0.0300.0120.013 0.0350.60 0.15